Directional self-assembly of organic vertically superposed nanowires

Precise spatial organization of organic micro/nanostructures is crucial for integrated optoelectronics, where low-dimensional organic crystal heterostructures (multiblock, core/shell, branched, and network types) have enabled advances in photonic and electronic manipulation. Despite progress, existing strategies typically control only the growth sequence rather than the exact spatial configuration of layers, leading to uncertainty in superimposed positions and predominantly horizontal stacking, which limits vertical space utilization and 3D photonic integration. Fixed-position multilayer heterostructures would enable better control of exciton/photon transport, but random nucleation hinders deterministic stacking. Inspired by oriented nucleation at dislocations used to fix branch sites, the study aims to realize organic vertically superimposed heterostructures with fixed interlayer positions by leveraging structure-induced, oriented self-assembly. The authors propose using semi-wrapped TDP/THT core/shell microrods with an exposed core segment as preferential nucleation sites to guide epitaxial growth of a second layer at a defined position, enabling vertical superposition and tunable interlayer length ratios.

Prior work on organic low-dimensional heterostructures includes multiblock, core/shell, branched, and network architectures that facilitate integrated nanophotonics and charge/photon management. Coaxial heterostructures provide enlarged heterojunction areas, enabling enhanced photonic signal processing and improved carrier collection. Nonaxial branched and network structures better support multiport input/output needed for optical signal processing and WDM systems. Fabrication strategies such as one-pot hierarchical self-assembly via noncovalent interaction control, stepwise seeded epitaxial growth, physical vapor transport, inkjet printing, and photochromic methods have been developed. However, these methods generally fine-tune growth sequences without precisely fixing interlayer positions, resulting in uncertain superposition sites and mostly horizontal stacking, limiting vertical integration. Oriented nucleation at dislocation points has been used to control branch sites precisely, suggesting that structural features can define nucleation loci and motivate the present approach to fix superposition positions in multilayer heterostructures.

Materials and design: Electron-donating molecules HDP and BHT were cocrystallized with electron-deficient TCNB via charge-transfer interactions to form red-emitting TDP and green-emitting THT 1D microrods, respectively. Orbital calculations showed new MOs for both cocrystals (TDP HOMO/LUMO: −5.60/−3.70 eV; THT HOMO/LUMO: −5.86/−3.62 eV). Simulated crystallographic parameters: TDP (monoclinic, P21/c; a=7.3528 Å, b=7.1026 Å, c=29.5733 Å, α=90°, β=90.347°, γ=90°) and THT (triclinic, P-1; a=7.2363 Å, b=8.0530 Å, c=16.9259 Å, α=88.451°, β=89.127°, γ=87.740°). Both exhibit 1D growth along [100], confirmed by TEM/SAED and XRD, with anisotropic polarization patterns.

Stepwise seeded self-assembly: 1) Prepare TDP seed microrods by solvent evaporation. 2) Introduce THT to epitaxially grow on TDP to form core/shell (C/S) heterostructures. By controlling TDP:THT molar ratio, either all-wrapped (1:20) or semi-wrapped (1:10) C/S structures are obtained, where the semi-wrapped form retains an exposed TDP core segment (growth gap). 3) Use semi-wrapped C/S microrods as 1st-layer building blocks and add TDP again; homogeneous nucleation at the exposed gap leads to epitaxial growth of a TDP 2nd layer, forming vertically superimposed heterostructures (OSHs) with fixed interlayer positions along the c-axis.

Mechanism analysis: Lattice compatibility and surface/interface energy balance drive facet-selective growth. Lattice matching at TDP/THT interfaces: tip surfaces d(100)TDP=7.35 Å vs d(01-1)THT=7.23 Å (η1≈98.37%); side surfaces d(011)TDP=6.91 Å vs d(01-1)THT=7.19 Å (mismatch ≈4.05%, i.e., ≈95.95% matching); upper surfaces d(001)TDP=7.40 Å vs d(011)THT=7.34 Å (η3≈99.19%). Attachment energies of TDP surfaces favor initial nucleation at the high-energy (100) tip (Eattach{100}s=−65.37 kcal mol−1 > |Eattach(011)s|=−54.22 > Eattach{002}s=−24.60), promoting THT growth there and along sides; the upper (001) surface has the lowest attachment energy, leading to exposure if THT is insufficient, forming the semi-wrapped gap. The semi-wrapped C/S is a high-energy metastable structure that provides a preferential nucleation site for the second TDP layer.

Quantitative control: By adjusting the TDP:THT ratios and deposition volumes/timings during 1st-layer formation, the exposed core length is tuned, which linearly sets the 2nd-layer length fraction (examples: sequences of 100 μL TDP, 1500 μL THT, then 350 μL TDP yield 21.7%; or 100 μL TDP, 300 μL THT, then 650 μL TDP yield 95.3%).

Characterization: SEM, FM, bright-field microscopy, TEM/SAED for structure and interfaces; XRD for crystallinity; spatially resolved PL spectra and CIE coordinates; polarization mapping; time-resolved PL for energy transfer; custom optical microscopy to measure waveguiding with 380 nm excitation; extraction of optical loss coefficients via single-exponential fits. Energy transfer assessed via spectral overlap and lifetime changes indicating FRET.

Methods (precise solutions): TDP stock: 0.02 mmol TCNB (3.6 mg) + 0.02 mmol HDP (7.2 mg) in 10 mL DCM (2.0 mM), then mixed with 10 mL EtOH for deposition (60 s crystallization). THT stock: 0.02 mmol TCNB (3.6 mg) + 0.02 mmol BHT (4.6 mg) in 10 mL DCM (2.0 mM), then diluted with 10 mL EtOH for deposition. Semi-wrapped C/S: TDP layer first, then THT with TDP:THT volume ratio 1:10–1:20. OSHs: sequence of TDP (100 μL), THT (e.g., 1500 μL), then TDP (e.g., 350 μL) with 60 s crystallization intervals; solvent evaporation on quartz substrates.

• Semi-wrapped TDP/THT core/shell microrods with an exposed TDP core segment were reliably fabricated by controlling TDP:THT ratios (1:10 yields semi-wrapped; 1:20 yields all-wrapped). The exposed gap serves as a high-energy, preferential nucleation site.
• Oriented, homogeneous nucleation of TDP at the exposed gap produced vertically superimposed heterostructures (OSHs) with fixed interlayer sites along the c-axis; 1st layer: semi-wrapped C/S; 2nd layer: TDP microrod.
• Interface exhibits 100% lattice matching due to identical packing and orientation at the contact surface; facet-specific matching rates: tip η≈98.37%; upper η≈99.19%; side mismatch ≈4.05% (~95.95% matching). An ultralow lattice mismatch (<5%) is identified as a prerequisite for heterogeneous structure formation.
• The 2nd-layer length fraction is linearly tunable by the TDP:THT ratio during 1st-layer formation, covering 21.7% to 95.3% length ratios; increasing THT proportion reduces exposed core length and thus the 2nd-layer length.
• Optical properties: clear heterojunction PL signatures; PL peaks of THT at 560 nm and TDP at ~735–750 nm observed at respective regions; CIE coordinates confirm emission colors.
• Evidence of FRET from THT to TDP: THT PL lifetime decreased from 94.81 to 70.07 ns; TDP lifetime increased from 25.65 to 34.15 ns upon heterojunction formation; emission shifts consistent with energy transfer leading to orange emission in C/S regions.
• Single-crystal waveguiding: low optical-loss coefficients measured as RTHT=0.174 ± 0.003 dB/μm and RTDP=0.130 ± 0.004 dB/μm with 380 nm excitation; high photon efficiencies of OSHs calculated as 93.8% (Input I) and 89.2% (Input II).
• Excitation-position-dependent waveguiding across four output channels (two tips per layer) enables multichannel photonic barcodes. Passive and active (FRET-assisted) modes are observed depending on whether the 1st or 2nd layer is excited; polarization-resolved outputs provide additional coding dimensions.
• Structural analyses: TEM/SAED confirm 2nd-layer TDP grows along [100]; SAED of the 1st-layer long microrod shows diffraction complexity due to core/shell superposition; XRD shows characteristic peaks of both TDP and THT.

The study addresses the key challenge of deterministic control over superimposed positions in organic multilayer heterostructures by introducing semi-wrapped core/shell microrods as high-energy metastable templates. The exposed core region serves as a defined nucleation site that directs homogeneous nucleation and epitaxial growth of a second layer at a fixed location, overcoming random nucleation. High lattice compatibility between TDP and THT (matching ≥95%) facilitates oriented assembly and ensures coherent interfaces. The approach enables linear control of the interlayer length ratio and leverages anisotropic optical properties to achieve excitation-position-dependent waveguiding and polarization-resolved outputs. The resulting multichannel outputs function as photonic barcodes, demonstrating relevance to photonic encryption and integrated optoelectronic circuits. Overall, controlling nucleation via structurally engineered templates is shown to be an effective, potentially general strategy for constructing highly ordered organic superimposed heterostructures.

An oriented integration strategy was developed to fabricate organic vertically superimposed heterostructures with fixed interlayer positions by using semi-wrapped TDP/THT core/shell microrods as nucleation-directing templates. The method achieves 100% lattice matching at the interface and precise, linear tuning of the 2nd-layer length fraction (21.7%–95.3%) via control of the exposed core area. The OSHs exhibit excitation-position-dependent waveguiding and polarization-sensitive multichannel outputs, enabling photonic barcodes for information processing and encryption. This work demonstrates a generalizable route to control nucleation sites and spatial organization in organic heterostructures, advancing integrated optoelectronics. Future work could extend the material scope, explore more complex multilayer stacks and networked architectures, and integrate the structures into device-level photonic circuitry.

• Material compatibility constraint: The authors indicate that an ultralow lattice mismatch (<5%) is a prerequisite for constructing heterogeneous structures; thus, the approach requires material pairs with high lattice matching (≥95%) and similar packing, which may limit generality.
• Dependence on semi-wrapped metastable templates: The fixed-position superposition relies on creating a semi-wrapped core/shell with a defined exposed gap; systems that do not form such metastable structures may not be directly amenable.
• Demonstrations focus on TCNB-based cocrystals (TDP and THT); performance and assembly behavior across broader material systems were not evaluated in this work.